INHIBITION OF ENZYMES 463 



Perkins (1934) found both reversible and irreversible components in the 

 inhibition of papain, sufide reactivating partially and to an extent de- 

 termined by various factors. One might expect this to be generally true and 

 the degree of reversibility to depend on the concentration of the quinone, 

 the time of exposure, the pH and many other variables, as well as on the 

 reductant used. It is not known if thiols can directly remove quinones bound 

 to enzymes. An oxidizing mechanism has been thought responsible for the 

 inhibition by p-benzoquinones of intestinal phosphatase (Sizer, 1943 b), 

 cobra venom cholinesterase (Chaudhuri, 1950 b), homogentisate oxidase 

 (Schepartz, 1953), carbonic anhydrase (Chiba et al., 1954 a), and horsera- 

 dish peroxidase (Klapper and Hackett, 1963), but other workers obtaining 

 comparable results with other enzymes have preferred to assume a 1,4 

 addition type of mechanism, and indeed the evidence for either mechanism 

 is usually very inadequate or absent. Chaudhuri (1950 b) showed that 

 cholinesterase could be completely reactivated by cysteine, sulfide, or ascor- 

 bate, while Klapper and Hackett (1963) found some correlation between 

 the oxidizing potencies of a few quinones and their inhibitory activities 

 against peroxidase. On the other hand, Blagoveshchenskii and Sorokina 

 (1937) could obtain no reversal with cysteine of yeast proteinase inactivated 

 by p-benzoquinone, and Hochster (1955) could likewise not reactivate 

 xylose isomerase with glutathione. Most of these results are difficult to 

 interpret and, in general, do not prove or disprove any mechanism. Schop- 

 fer and Grob (1949 b) considered this problem in some detail with respect 

 to the inhibition of urease, and although they found some correlation be- 

 tween redox potentials and inhibition and showed reversal of 2-chloro- 

 1,4-naphthoquinone-inactivated enzyme, they correctly concluded that one 

 could not adequately distinguish between oxidation and 1,4 addition. 



Reaction of Quinones with Enzyme SH Groups 



Although much has been written of the likelihood of reactions between 

 the quinones and enzyme SH groups, and the quinones have often been 

 classified as SH reagents, it is surprising and disconcerting to find very 

 little experimental evidence for such a mechanism of enzyme inhibition. 

 The concept of the quinones as SH reagents was not seriously considered 

 until the work of Potter and DuBois (1943) on succinate dehydrogenase; 

 it is quite sensitive to a variety of quinones, and the combination with the 

 active site SH group is represented as 



OH 



Enzyme 



